The invention relates to a thermoelectronic energy converter device having an emitter electrode, which is capable of emitting electrons in response to an application of thermal energy, and a collector electrode, which is capable of collecting the emitted electrons. In particular, the invention relates to a thermoelectronic energy converter device being configured as a power source converting heat into a consumable electric current or as a heat sink (or heat source) device converting electric energy into heat. Furthermore, the present invention relates to a method of thermoelectronic energy conversion of thermal energy to electric energy or vice versa, wherein the thermoelectronic energy converter device is used. Applications of the invention are available in the field of generating electric power, in particular on the basis of solar energy or thermal energy from nuclear reactions, or in the field of cooling or heating devices.
Thermoelectronic generators, which produce electric power directly from a temperature gradient are generally known as sources of electricity provided from solar energy (see e.g. Y. G. Yeng et al. in “Journal of Renewable and Sustainable Energy”, Vol. 1, 2009, p. 052701; G. P. Smestad in “Solar Energy Materials and Solar Cells”, Vol. 82, 2004, p. 227; J. W. Schwede et al. in “Nature Materials”, Vol. 9, 2010, p. 762; and U.S. Pat. No. 6,313,391) or nuclear decay (see e.g. N. S. Rasor et al. in “Atomics International”, Canoga Park, 1962). These generators—in the literature usually referred to as thermionic generators—produce the electric power directly from a temperature difference between an electron emitter and an electron collector, which are spaced by an evacuated gap. By applying thermal energy, e.g. focused solar radiation, the temperature of the electron emitter is increased so that electrons having an energy above the work function of the electron emitter material can be released into the free space. The emitted electrons travel through the evacuated gap to the electron collector, which is operated at a temperature lower than the electron emitter temperature. The electrons condense on the electron collector, and the electron collector becomes negatively charged with respect to the electron emitter. Accordingly, the thermoelectronic generator can act as a source of electric power, which can be harvested e.g. by connecting the electron emitter and the electron collector through a load circuit. Because thermoelectronic generators can in principle be operated at very high temperatures, e.g. emitter temperatures above 1500° C., with very large temperature differences between the electron emitter and the electron collector, and because heat loss can in principle be very small, high conversion efficiencies have been predicted for the thermoelectronic generators in the literature (see e.g. J. H. Ingold in “Journal of Applied Physics”, Vol. 32, 1961, p. 769).
As a general problem, the emission of electrons from the electron emitter is usually limited by space charges, which are built-up near the electron emitter surface. The released electrons form an electron cloud, thus providing a barrier against the emission of further electrons. These space charges drastically limit the current of emitted electrons and therefore the power generated by the thermoelectronic generator. Conventionally, three techniques have been developed for suppressing the space charge effects.
Firstly, it has been realized to decrease or even neutralize the space charge cloud by an injection of positively charged ions, like e.g. Cs ions. However, this approach has essential drawbacks. The ions have to be generated in a power consuming process. Furthermore, the ions have to be injected into the space charge region at a desired density, while chemical reactions and a condensation of the ions at undesirable positions have to be avoided. As a further disadvantage, for ensuring a long lifetime of the generator, the ions have to be recycled. Finally, energy is lost by undesired electron-ion collisions and heat transport by ion gas.
As an example of the first approach, U.S. Pat. No. 3,267,307 discloses a thermionic generator using Cs ions decreasing the space charge cloud. For reducing the heat transport, a permeable heat shield is provided in the Cs vapor filled gap between the emitter to the collector. The heat shield has a complex tube or foil structure being made of conducting and insulating materials and including openings through which electrons travel from the emitter to the collector. A magnetic field is provided for concentrating the electrons along paths through the openings. For avoiding a deposition of Cs ions on the heat shield, a small bias voltage is applied to the heat shield.
According to a second approach, emitted electrons are accelerated by an electric field, which is created by an additional electrode (anode or acceleration electrode). A positive voltage is applied to the anode, so that electrons are accelerated out of the space charge cloud. As an example of the second approach, U.S. Pat. No. 3,477,012 discloses a thermoelectronic generator with a coaxial structure having a central emitter rod surrounded by a hollow cylindrical anode and an outer cylindrical collector. By the effect of a magnetic field, electrons released from the emitter and accelerated to the anode are deflected to an exposed inner surface of the collector. Although the gap between the emitter and the collector is evacuated and the use of Cs ions can theoretically be avoided with this technique, there are disadvantages in terms of the complex structure, restricted scalability and limited energy conversion efficiency of the conventional thermoelectronic generator. Since the electric field and the trajectories of the electrons along the magnetic field lines are perpendicular, the electric field does not accelerate electrons towards the collector. Hence, it does not decrease the space charge, which leads to small efficiencies.
It is also known that acceleration electrodes are used in science and technology on a regular basis. One such usage, the experimental investigations of the electron release at the emitter with local resolution, has been described by George N. Hatsopoulos in “Thermionic energy conversion” (vol. 2, U.S. Dept. of Energy, 1979, p. 491 to 493). This acceleration electrode is a plane plate being arranged between the emitter and the collector and having one aperture through which electrons can pass. An efficient energy conversion was excluded with this experimental set-up.
A third approach is based on the fabrication of the generator with emitter-collector distances too small for space charges to form (see e.g. J.-H. Lee et al. in “Appl. Phys. Lett.” vol. 100, 2012, p. 173904). This is usually referred to as “close-space-technique”. However, this concept has a serious drawback in terms of the necessary stabilization of practically large area electron emitter and electron collector surfaces at large temperature differences with a precision of micrometers or fractions thereof. The stabilized emitter-collector distance in particular has to be kept constant during eventual thermal expansions of the components. Today it is expected that the only concept for avoiding this drawback is the above Cs ion based first approach as mentioned by J.-H. Lee et al. in 2012.
Despite of the drawbacks of the conventional techniques, thermoelectronic generators have been used in several Russian space crafts, wherein the electron emitter has been heated with radioisotopes and the space charge has been suppressed by the above Cs approach. On the other hand, the close-space-technique has never been industrially applied.
The objective of the invention is to provide an improved thermoelectronic energy converter device and an improved method of thermoelectronic energy conversion, respectively, wherein disadvantages and limitations of conventional techniques are avoided. In particular, the thermoelectronic energy conversion is to be obtained with increased efficiency, improved reliability and/or reduced complexity of the device structure.
These objectives are solved with a thermoelectronic energy converter device and a method for thermoelectronic energy conversion of the invention.